Free cutting alloys



Patented Apr. 13, 1937 UNITED STATES FREE CUTTING ALLOYS Louis W. Kempf and Walter A. Dean. Cleveland,

Ohio, assignors to America, Pittsburgh,

Pennsylvania Aluminum Company of a corporation of No Drawing. Application December 28, 1935, Serial No. 56,545

Claims.

The invention relates to aluminum base alloys and is particularly concerned with aluminum base alloys containing copper, tin and silicon.

Aluminum base alloys containing between 3 5 per cent and 12 per cent of copper have been widely used heretofore. The copper imparts good casting characteristics and increases the tensile strength, yield strength, and hardness of the alloy. In the lower portion of the copper range the alloy may be mechanically deformed by the well known commercial processes such as rolling,

,iforging, drawing, or extrusion. Alloys containing more than about 6 per cent copper are generally used in the cast condition. Through- 15 out the entire range of 3 to 12 per cent copper, however, the alloys are susceptible to variations and improvements in their physical properties by thermal treatments.

The physical properties of the foregoing type of aluminum-copper alloy in the solution heat treated and aged conditions are greatly improved by the addition of from 0.005 to 0.1 per cent of .tin as described in co-pending application No. 606,756 and U. S. Patent 2,022,686, issued December 3, 1935. The small amount of tin is particularly useful in increasing the yield strength of the heat treated and aged alloy.' The use of tin in the aforesaid proportions does not substantially change the working characteristics of the alloy whereas special precautions must be taken to roll or forge alloys containing a larger amount of tin. The addition of from 0.5 to 2 per cent of silicon to the aluminum-copper-tin alloy described above serves to improve the casting and working qualities of the alloy. Be-

cause of the high strength which is attainable in these alloys when heat treated and aged, they are generally used in this condition. Our invention which is describedhereinbelow, is directed to improving the machining quality of such alloys. Since all commercial aluminum base alloys contain some silicon as an impurity, usually less than, or not greatly in excess of, 0.5 per cent, it is to be understood that the amount of silicon discussed herein, and hereinafter claimed, represents the total quantity of silicon present, and that the silicon content of I the aluminum used should be known in order to provide a basis for determining the eventual composition of the aluminum-copper-tin-silicon alloy.

There are, however, some applications wherein aluminum-copper-tin-silicon alloys as hereinabove disclosed might be conveniently and profitably used except for an inherent disadvantage which militates against their use in the production of certain articles requiring exacting machining operations. Mechanical cutting operations such as drilling, shaping, or lathe- 60 cutting are successfully carried out only by using certain precautions which increase the cost of production and perhaps favor the choice of another metal or alloy which can.be machined more readily but whichis not so desirable in other respects, as for example, in physical properties. When alloys are diflicult'to'machine this disadvantage becomes ewdent, in many cases, through rapid wear of the cutting tool edge, so that frequent tool re-sharpening is'requir'ed. Despite continual lubrication the machined surface is rough and irregular, and'the chip has a tendency to form a continuous curl or spiral which often fouls the tool or the moving parts of the machine. It is immediately apparent that there is need for an alloy of good working characteristics and satisfactory physical properties,

yet possessing such favorable machining properties that the complete machining operations may be performed economically and successfully, and may be productive of a pleasing surface appearance.

Accordingly an object of our invention is the provision of an aluminum base alloy containing from about 3 per cent to about 12 per cent of copper, from about 0.005 to about 0.1 per cent of tin, and from about 0.5 per cent to about 2.0 per cent of silicon which may be readily and economically machined.

Our invention resides in the discovery that the foregoing object is effected by the addition of two or more of the elements lead-thallium, cadmium or bismuth. The aluminum-copper-tinsilicon alloys to which these elements are added in the proportions specified below, are known as free cutting or free machining alloys because they can be machined more rapidly than similar alloys without these elements and yet have assurface. After an I good or a better finished extended series of investigations we covered that these four metals when added to aluminum-copper-tin-silicon alloys, form a class of alloying elements by reason of their favorable effect upon the machining properties of. these alloys. In recognition of this effect we term lead, thallium, cadmium and bismuth free machining elements. We have further discovered that the simultaneous presence of two or more of these elements is productive of an improvement in free machining characteristics which is considerably greater than that caused by the presence of the same total amount of a single free machining element. For example, the addition of 0.5 per cent oflead and 0.5 per cent of bismuth to an aluminum base alloy containing about 5 per cent of copper, 0.05 per cent of tin, and 1.25 per cent of silicon effects a greater improvement in machining quality than does the addition of 1.0 per cent of either lead or bismuth singly.

have dis- These four elements, we believe, are uniquewith respect to their effect on the machining characteristics of aluminum-copper-tin-silicon alloys. It is a fortunate circumstance, therefore, that they are also of relatively low melting point, a fact which makes possible their addition to molten aluminum in the pure state, without the intervention of so-called rich alloys. As a matter of fact, we have observed that of all the metals whose melting point is lower than about 327 C., the melting point of lead, the four elements we have selected are the only ones, with the exception of tin which is not desired in this alloy in free cutting amounts which are commercially suitable and which impart free cutting characteristics but do not have undesirable effect on the fundamental physical properties of the base alloy;

The total amount of free machining elements should not be less than about 0.05 per cent since below this amount there is scarcely any advantageous effect. We have determined that a maximum limit of about 6 per cent total of two or more of the free machining elements is sufficient for satisfactory commercial results, since although the free machining effect persists beyond this amount, certain of the other physical properties may be unfavorably affected.

Aluminum-copper-tin-silicon alloys containing two or more of the free machining elements lead, thallium, cadmium and bismuth may be machined more rapidly, with less tool wear, less tool sharpening, better quality of chip and better machined surface than the same base alloys without .the free machining additions, and in fact better than the same base alloys containing an equivalent total amount of a single free machin- -ing element.

Since aluminum base alloys containing from 3 to- 12 per cent of copper have a wide variety o l-applications we list several alloys each of which may be said to be preferred for a particular purpose. As an alloy for mechanical de formation we suggest an aluminum base alloy containing 5 per cent of copper, 0.05 per cent of tin, 1.25 per cent of silicon, and a total of 1 per cent of free cutting constituents, for example, 0.5 per cent of bismuth and 0.5 per cent of cadmium, the balance being aluminum. For an alloy with excellent casting characteristics to be used in the unworked condition we suggest an alloy containing per cent of copper, 0.05 per cent of 'tin, 1.5 per cent of silicon and a total of 3 per cent of free machining elements, the balance being aluminum.

For certain purposes, notably the improvement of tensile strength, hardness and grain structure, the alloys as hereinabove disclosed may be improved by the addition of one or more of the group of elements composed of molybdenum, vanadium, titanium, tungsten, zirconium and chromium. From 0.05 to 1 per cent of any one of these elements may be used alone, but if more than one is employed the total amount should not exceed about 2 per cent.

It is characteristic of the elements lead, tin,

thallium, cadmium, and bismuth that they form with aluminum a series of alloys of limited liquid solubility. We have reason to believe that the free machining elements are the only elements which exhibit this characteristic, with the possible exception of several metals which are not regarded as having any commercial promise as additions to aluminum base alloys. Within the range disclosed and claimed however the free machining elements may be added without unusual difllculty. We suspect that this characteristic feature of the disclosed elements may be one of the significant factors which contribute to their free machining effect. We believe that this effect is further strengthened by distributing the free machining constituent relatively homogeneously throughout the solid matrix, since these free machining constituents are also practically insoluble in the solid aluminum base.

Thefree machining alloys which have been described hereinabove may be subjected to the thermal treatments well known in the art to improve their strength and hardness. We have found that a solution heat treatment and subsequent aging does not impair the free machining quality of the alloys and in many instances the treatment even tends to improve this property. For many purposes a relatively high strength and hardness are necessary to the successful performance of the machined article and hence the alloy must be heat treated. This treatment is generally applied prior to the machining'operation.

As hereinabove indicated the free machining elements, by reason of their low melting point, may be added to the molten aluminum alloy in pure metallic form. However, since some difficulty may be encountered in introducing them in the higher percentages of our disclosed range we prefer to use themethod which is more fully described in U. S. Patent No. 1,959,029, issued March 15, 1934. Briefly it involves heating the melt to a somewhat higher temperature than is customary, and vigorously stirring it in excess of a critical period of time.

The term aluminum as used herein and in the appended claims embraces the usual impurities found in aluminum ingot of commercial grade or picked up in the course of the ordinary handling operations incident to melting practice.

We claim:

-1. An aluminum base alloy consisting of about 5 per cent copper, 0.05 per cent tin, 1.25 per cent silicon, 0.5 per cent lead and 0.5 per cent bismuth, the balance being aluminum.

2. An aluminum base alloy consisting of about 5 per cent copper, 0.05 per cent tin, 1.25 per cent silicon, 0.5 per cent bismuth and 0.5 per cent cadmium, the balance being aluminum.

3. A free cutting alloy containing from 3 to 12 per cent of copper, from'0.005 to 0.1 per cent of tin, 0.5 to 2 per cent of from 0.05 to' 6 percent of at least two of the elements from the following metals, lead, thallium, cadmium, and bismuth, to improve its machining properties, the balance being substantially aluminum.

4. A free cutting alloy containing from 3 to 12 per cent of copper, from 0.005 to 0.1 per cent of tin, from 0.5 to 2 per cent of silicon, from 0.05 to 2 per cent of hardening metal from the group composed of molybdenum, vanadium, titanium, tungsten, zirconium and chromium,

,and a total of from 0.05 to 6 per cent of at least two of the elements from the following metals, lead, thallium, cadmium, and bismuth, to improve its machining properties, the balance being substantially aluminum. 1

5. An aluminum base alloy consisting of about 5 per cent copper, 0.05 per cent tin, 1.25 per cent silicon, 0.5 per cent cadmium, lead, the balance being aluminum.

LOUIS W. KEMPF. WALTER A. DEAN.

silicon, and a total of,

and 0.5 per cent 

